1. Field of the Invention
This invention relates to a homogeneous catalysis process that employs as a catalyst a gaseous transition metal compound subjected to photo-induced deligandation.
2. Description of the Prior Art
A known method of preparing catalysts is by deligandation; i.e., the removal of one or more ligands, of compounds of transition metals and ligands. Deligandation has generally been accomplished either thermally or photochemically in solution.
Thermal deligandation of a compound of the form ML.sub.n, where M is a transition metal and L is a ligand, generally results in the production of a mixture of species (ML.sub.n-1, ML.sub.n-2, . . . ) in which the more highly active species are relatively minor constituents. Although removal of a single ligand and complete removal of ligands have been reported, the removal of a specified number of ligands (more than one) cannot in general be accomplished thermally.
Photochemical deligandation, particularly of transition metal carbonyls, has been the subject of a great deal of study and has been summarized recently in G. L. Geoffroy and M. S. Wrighton, Organometallic Photochemistry (Academic Press, New York, 1979), Chapter 2. Dissociative loss of one or more CO groups yields one or more coordinatively unsaturated intermediates. In general, the photo reactions have been accomplished with the transition metal carbonyl compounds either in an inert liquid solvent (see, e.g., M. Wrighton, Chem. Rev. 74, 401 (1974)) or matrix-isolated; i.e., trapped in an inert rigid matrix, usually a solid noble gas at low temperatures (see, e.g., J. J. Turner et al., Pure and Appl. Chem. 49, 271 (1977)).
Photochemical treatment of transition metal carbonyls in solution generally results in loss of only the single ligand that is most weakly bound, while irradiation of matrix-isolated carbonyls requires high input energy and yields a deligandated product in a frozen form, of limited direct utility.
Photolysis of metal carbonyls in the gaseous phase has been reported by Z. Karny et al., Chem. Phys. Lett. 59, 33 (1978) and by L. Hellner et al., Nouveau Journal De Chimie 3, 721 (1979). Karny et al. produced electronically excited metal atoms by focusing on the gas the output of an ArF (193 nm) or KrF (249 nm) laser. The fluence incident on the gas was extremely high. The production of excited metal atoms was postulated to occur by a multiphoton mechanism in which the net yield of excited metal atoms depends on the square of the radiation intensity. Hellner et al., on the other hand, used rather low fluence from conventional rare gas lamps. While Karny et al. gave no indication that partial deligandation results from their procedure, Hellner et al. specifically concluded that their process involves direct dissociation to an electronically excited metal atom without generating any intermediate products. The resultant excited metal atom is generally not a good catalyst.
Transition metal compounds, including carbonyls, have been studied as photocatalysts and photoassistance agents (M. S. Wrighton et al. Pure and Appl. Chem. 41, 671 (1975)). Photolysis of W(CO).sub.6, Mo(CO).sub.6, Cr(CO).sub.6 and Fe(CO).sub.5 in liquid solvents leads to the generation of intermediates that assist or catalyze olefin reactions, including isomerization.